# Optimistic Sync ## Introduction In order to provide a syncing execution engine with a partial view of the head of the chain, it may be desirable for a consensus engine to import beacon blocks without verifying the execution payloads. This partial sync is called an *optimistic sync*. Optimistic sync is designed to be opt-in and backwards compatible (i.e., non-optimistic nodes can tolerate optimistic nodes on the network and vice versa). Optimistic sync is not a fundamental requirement for consensus nodes. Rather, it's a stop-gap measure to allow execution nodes to sync via established methods until future Ethereum roadmap items are implemented (e.g., statelessness). ## Constants |Name|Value|Unit |---|---|---| |`SAFE_SLOTS_TO_IMPORT_OPTIMISTICALLY`| `128` | slots *Note: the `SAFE_SLOTS_TO_IMPORT_OPTIMISTICALLY` must be user-configurable. See [Fork Choice Poisoning](#fork-choice-poisoning).* ## Helpers For brevity, we define two aliases for values of the `status` field on `PayloadStatusV1`: - Alias `NOT_VALIDATED` to: - `SYNCING` - `ACCEPTED` - Alias `INVALIDATED` to: - `INVALID` - `INVALID_BLOCK_HASH` - `INVALID_TERMINAL_BLOCK` Let `head: BeaconBlock` be the result of calling of the fork choice algorithm at the time of block production. Let `head_block_root: Root` be the root of that block. Let `blocks: Dict[Root, BeaconBlock]` and `block_states: Dict[Root, BeaconState]` be the blocks (and accompanying states) that have been verified either completely or optimistically. Let `optimistic_roots: Set[Root]` be the set of `hash_tree_root(block)` for all optimistically imported blocks which have only received a `NOT_VALIDATED` designation from an execution engine (i.e., they are not known to be `INVALIDATED` or `VALID`). Let `current_slot: Slot` be `(time - genesis_time) // SECONDS_PER_SLOT` where `time` is the UNIX time according to the local system clock. ```python @dataclass class OptimisticStore(object): optimistic_roots: Set[Root] head_block_root: Root blocks: Dict[Root, BeaconBlock] block_states: Dict[Root, BeaconState] ``` ```python def is_optimistic(opt_store: OptimisticStore, block: BeaconBlock) -> bool: return hash_tree_root(block) in opt_store.optimistic_roots ``` ```python def latest_verified_ancestor(opt_store: OptimisticStore, block: BeaconBlock) -> BeaconBlock: # It is assumed that the `block` parameter is never an INVALIDATED block. while True: if not is_optimistic(opt_store, block) or block.parent_root == Root(): return block block = opt_store.blocks[block.parent_root] ``` ```python def is_execution_block(block: BeaconBlock) -> bool: return block.body.execution_payload != ExecutionPayload() ``` ```python def is_optimistic_candidate_block(opt_store: OptimisticStore, current_slot: Slot, block: BeaconBlock) -> bool: if is_execution_block(opt_store.blocks[block.parent_root]): return True if block.slot + SAFE_SLOTS_TO_IMPORT_OPTIMISTICALLY <= current_slot: return True return False ``` Let only a node which returns `is_optimistic(opt_store, head) is True` be an *optimistic node*. Let only a validator on an optimistic node be an *optimistic validator*. When this specification only defines behaviour for an optimistic node/validator, but *not* for the non-optimistic case, assume default behaviours without regard for optimistic sync. ## Mechanisms ### When to optimistically import blocks A block MAY be optimistically imported when `is_optimistic_candidate_block(opt_store, current_slot, block)` returns `True`. This ensures that blocks are only optimistically imported if one or more of the following are true: 1. The parent of the block has execution enabled. 1. The current slot (as per the system clock) is at least `SAFE_SLOTS_TO_IMPORT_OPTIMISTICALLY` ahead of the slot of the block being imported. In effect, there are restrictions on when a *merge block* can be optimistically imported. The merge block is the first block in any chain where `is_execution_block(block) == True`. Any descendant of a merge block may be imported optimistically at any time. *See [Fork Choice Poisoning](#fork-choice-poisoning) for the motivations behind these conditions.* ### How to optimistically import blocks To optimistically import a block: - The [`notify_new_payload`](../specs/bellatrix/beacon-chain.md#notify_new_payload) function MUST return `True` if the execution engine returns `NOT_VALIDATED` or `VALID`. An `INVALIDATED` response MUST return `False`. - The [`validate_merge_block`](../specs/bellatrix/fork-choice.md#validate_merge_block) function MUST NOT raise an assertion if both the `pow_block` and `pow_parent` are unknown to the execution engine. - All other assertions in [`validate_merge_block`](../specs/bellatrix/fork-choice.md#validate_merge_block) (e.g., `TERMINAL_BLOCK_HASH`) MUST prevent an optimistic import. - The parent of the block MUST NOT have an `INVALIDATED` execution payload. In addition to this change in validation, the consensus engine MUST track which blocks returned `NOT_VALIDATED` and which returned `VALID` for subsequent processing. Optimistically imported blocks MUST pass all verifications included in `process_block` (withstanding the modifications to `notify_new_payload`). A consensus engine MUST be able to retrospectively (i.e., after import) modify the status of `NOT_VALIDATED` blocks to be either `VALID` or `INVALIDATED` based upon responses from an execution engine. I.e., perform the following transitions: - `NOT_VALIDATED` -> `VALID` - `NOT_VALIDATED` -> `INVALIDATED` When a block transitions from `NOT_VALIDATED` -> `VALID`, all *ancestors* of the block MUST also transition from `NOT_VALIDATED` -> `VALID`. Such a block and any previously `NOT_VALIDATED` ancestors are no longer considered "optimistically imported". When a block transitions from `NOT_VALIDATED` -> `INVALIDATED`, all *descendants* of the block MUST also transition from `NOT_VALIDATED` -> `INVALIDATED`. When a block transitions from the `NOT_VALIDATED` state, it is removed from the set of `opt_store.optimistic_roots`. When a "merge block" (i.e. the first block which enables execution in a chain) is declared to be `VALID` by an execution engine (either directly or indirectly), the full [`validate_merge_block`](../specs/bellatrix/fork-choice.md#validate_merge_block) MUST be run against the merge block. If the block fails [`validate_merge_block`](../specs/bellatrix/fork-choice.md#validate_merge_block), the merge block MUST be treated the same as an `INVALIDATED` block (i.e., it and all its descendants are invalidated and removed from the block tree). ### Execution Engine Errors When an execution engine returns an error or fails to respond to a payload validity request for some block, a consensus engine: - MUST NOT optimistically import the block. - MUST NOT apply the block to the fork choice store. - MAY queue the block for later processing. ### Assumptions about Execution Engine Behaviour This specification assumes execution engines will only return `NOT_VALIDATED` when there is insufficient information available to make a `VALID` or `INVALIDATED` determination on the given `ExecutionPayload` (e.g., the parent payload is unknown). Specifically, `NOT_VALIDATED` responses should be fork-specific, in that the search for a block on one chain MUST NOT trigger a `NOT_VALIDATED` response for another chain. ### Re-Orgs The consensus engine MUST support any chain reorganisation which does *not* affect the justified checkpoint. If the justified checkpoint transitions from `NOT_VALIDATED` -> `INVALIDATED`, a consensus engine MAY choose to alert the user and force the application to exit. ## Fork Choice Consensus engines MUST support removing blocks from fork choice that transition from `NOT_VALIDATED` to `INVALIDATED`. Specifically, a block deemed `INVALIDATED` at any point MUST NOT be included in the canonical chain and the weights from those `INVALIDATED` blocks MUST NOT be applied to any `VALID` or `NOT_VALIDATED` ancestors. ### Fork Choice Poisoning During the merge transition it is possible for an attacker to craft a `BeaconBlock` with an execution payload that references an eternally-unavailable `body.execution_payload.parent_hash` (i.e., the parent hash is random bytes). In rare circumstances, it is possible that an attacker can build atop such a block to trigger justification. If an optimistic node imports this malicious chain, that node will have a "poisoned" fork choice store, such that the node is unable to produce a block that descends from the head (due to the invalid chain of payloads) and the node is unable to produce a block that forks around the head (due to the justification of the malicious chain). If an honest chain exists which justifies a higher epoch than the malicious chain, that chain will take precedence and revive any poisoned store. Such a chain, if imported before the malicious chain, will prevent the store from being poisoned. Therefore, the poisoning attack is temporary if >= 2/3rds of the network is honest and non-faulty. The `SAFE_SLOTS_TO_IMPORT_OPTIMISTICALLY` parameter assumes that the network will justify a honest chain within some number of slots. With this assumption, it is acceptable to optimistically import transition blocks during the sync process. Since there is an assumption that an honest chain with a higher justified checkpoint exists, any fork choice poisoning will be short-lived and resolved before that node is required to produce a block. However, the assumption that the honest, canonical chain will always justify within `SAFE_SLOTS_TO_IMPORT_OPTIMISTICALLY` slots is dubious. Therefore, clients MUST provide the following command line flag to assist with manual disaster recovery: - `--safe-slots-to-import-optimistically`: modifies the `SAFE_SLOTS_TO_IMPORT_OPTIMISTICALLY`. ## Checkpoint Sync (Weak Subjectivity Sync) A consensus engine MAY assume that the `ExecutionPayload` of a block used as an anchor for checkpoint sync is `VALID` without necessarily providing that payload to an execution engine. ## Validator assignments An optimistic node is *not* a full node. It is unable to produce blocks, since an execution engine cannot produce a payload upon an unknown parent. It cannot faithfully attest to the head block of the chain, since it has not fully verified that block. ### Block Production An optimistic validator MUST NOT produce a block (i.e., sign across the `DOMAIN_BEACON_PROPOSER` domain). ### Attesting An optimistic validator MUST NOT participate in attestation (i.e., sign across the `DOMAIN_BEACON_ATTESTER`, `DOMAIN_SELECTION_PROOF` or `DOMAIN_AGGREGATE_AND_PROOF` domains). ### Participating in Sync Committees An optimistic validator MUST NOT participate in sync committees (i.e., sign across the `DOMAIN_SYNC_COMMITTEE`, `DOMAIN_SYNC_COMMITTEE_SELECTION_PROOF` or `DOMAIN_CONTRIBUTION_AND_PROOF` domains). ## Ethereum Beacon APIs Consensus engines which provide an implementation of the [Ethereum Beacon APIs](https://github.com/ethereum/beacon-APIs) must take care to avoid presenting optimistic blocks as fully-verified blocks. ### Helpers Let the following response types be defined as any response with the corresponding HTTP status code: - "Success" Response: Status Codes 200-299. - "Not Found" Response: Status Code 404. - "Syncing" Response: Status Code 503. ### Requests for Optimistic Blocks When information about an optimistic block is requested, the consensus engine: - MUST NOT respond with success. - MAY respond with not found. - MAY respond with syncing. ### Requests for an Optimistic Head When `is_optimistic(opt_store, head) is True`, the consensus engine: - MUST NOT return an optimistic `head`. - MAY substitute the head block with `latest_verified_ancestor(block)`. - MAY return syncing. ### Requests to Validators Endpoints When `is_optimistic(opt_store, head) is True`, the consensus engine MUST return syncing to all endpoints which match the following pattern: - `eth/*/validator/*` ## Design Decision Rationale ### Why `SAFE_SLOTS_TO_IMPORT_OPTIMISTICALLY`? Nodes can only import an optimistic block if their justified checkpoint is verified or the block is older than `SAFE_SLOTS_TO_IMPORT_OPTIMISTICALLY`. These restraints are applied in order to mitigate an attack where a block which enables execution (a *transition block*) can reference a junk parent hash. This makes it impossible for honest nodes to build atop that block. If an attacker exploits a nuance in fork choice `filter_block_tree`, they can, in some rare cases, produce a junk block that out-competes all locally produced blocks for the head. This prevents a node from producing a chain of blocks, therefore breaking liveness. Thankfully, if 2/3rds of validators are not poisoned, they can justify an honest chain which will un-poison all other nodes. Notably, this attack only exists for optimistic nodes. Nodes which fully verify the transition block will reject a block with a junk parent hash. Therefore, liveness is unaffected if a vast majority of nodes have fully synced execution and consensus clients before and during the transition. Given all of this, we can say two things: 1. **BNs which are following the head during the transition shouldn't optimistically import the transition block.** If 1/3rd of validators optimistically import the poison block, there will be no remaining nodes to justify an honest chain. 2. **BNs which are syncing can optimistically import transition blocks.** In this case a justified chain already exists blocks. The poison block would be quickly reverted and would have no affect on liveness. Astute readers will notice that (2) contains a glaring assumption about network liveness. This is necessary because a node cannot feasibly ascertain that the transition block is justified without importing that block and risking poisoning. Therefore, we use `SAFE_SLOTS_TO_IMPORT_OPTIMISTICALLY` to say something along the lines of: *"if the transition block is sufficiently old enough, then we can just assume that block is honest or there exists an honest justified chain to out-compete it."* Note the use of "feasibly" in the previous paragraph. One can imagine mechanisms to check that a block is justified before importing it. For example, just keep processing blocks without adding them to fork choice. However, there are still edge-cases here (e.g., when to halt and declare there was no justification?) and how to mitigate implementation complexity. At this point, it's important to reflect on the attack and how likely it is to happen. It requires some rather contrived circumstances and it seems very unlikely to occur. Therefore, we need to consider if adding complexity to avoid an unlikely attack increases or decreases our total risk. Presently, it appears that `SAFE_SLOTS_TO_IMPORT_OPTIMISTICALLY` sits in a sweet spot for this trade-off. ### Transitioning from VALID -> INVALIDATED or INVALIDATED -> VALID These operations are purposefully omitted. It is outside of the scope of the specification since it's only possible with a faulty EE. Such a scenario requires manual intervention. ## What about Light Clients? An alternative to optimistic sync is to run a light client inside/alongside beacon nodes that mitigates the need for optimistic sync by providing tip-of-chain blocks to the execution engine. However, light clients comes with their own set of complexities. Relying on light clients may also restrict nodes from syncing from genesis, if they so desire. A notable thing about optimistic sync is that it's *optional*. Should an implementation decide to go the light-client route, then they can just ignore optimistic sync all together. ## What if `TERMINAL_BLOCK_HASH` is used? If the terminal block hash override is used (i.e., `TERMINAL_BLOCK_HASH != Hash32()`), the [`validate_merge_block`](../specs/bellatrix/fork-choice.md#validate_merge_block) function will deterministically return `True` or `False`. Whilst it's not *technically* required retrospectively call [`validate_merge_block`](../specs/bellatrix/fork-choice.md#validate_merge_block) on a transition block that matches `TERMINAL_BLOCK_HASH` after an optimistic sync, doing so will have no effect. For simplicity, the optimistic sync specification does not define edge-case behaviour for when `TERMINAL_BLOCK_HASH` is used.